The crankshaft of an internal combustion engine mounted on a motor vehicle as a prime mover thereof, is arranged to drive several auxiliary power consuming units, such as a dynamo (alternator) and a cooling fan by means of a belt which is usually called a fan belt. Although the tension of the fan belt is so adjusted that all of the auxiliary power consuming units are properly driven by the engine, the tension along the fan belt is apt to be reduced. With insufficient tension the fan belt usually slips so that the engine power is not properly transmitted to the auxiliary power consuming units. As the result, tendency to undesirable phenomena, such as overheating the engine and overdischarge of the battery, increases.
There are some conventional detection devices which detect the slipping operation or rate of the fan belt. For instance, according to two Japanese Patent applications (provisional publication numbers: 47-41680, and 50-75468), the A.C. voltage generated by the alternator driven by the engine is detected and the frequency of the A.C. voltage is converted into an analogue signal the voltage of which is proportional to the frequency. Meanwhile, a pulse signal synchronous with the crankshaft rotation is detected and the frequency of the pulse signal is converted into a second analogue signal the voltage of which is proportional to the frequency. The voltages of the first and second analogue signals are compared by means of a comparator or a differential amplifier to see the difference therebetween. Since the difference between the first and second signals indicates the slipping rate of the fan belt, the output signal of the comparator or the differential amplifier may be used for issuing an alarm signal.
However, the above mentioned prior art technique, suffers from the following disadvantages. Since the first information derived from the alternator and the second information derived from the engine crankshaft rotation are respectively converted into analogue signals, the voltages of these analogue signals vary in wide ranges as the engine crankshaft rotational speed varies. When the input voltages of the comparator or the differential amplifier vary in a wide range, the output signal of the comparator or the differential amplifier is apt to include errors. In other words, accurate detection of the difference between the first and second pieces of information can not be obtained when the engine r.p.m. changes drastically. Furthermore, since the variation range of the first and second analogue signals are so wide, the dynamic range of the comparator or the differential amplifier has to be large enough. This causes the detection device to be expensive. It will be understood that the engine speed for a motor vehicle especially varies in a considerably wide range, and therefore, the above mentioned disadvantages are inevitable.